Cold-drawing is a well-known phenomenon in both glassy amorphous and semicrystalline polymers, which is seldom observed in rubber-like networks above the glass transition temperature. Cold-drawing and neck formation have recently been reported in smectic main-chain liquid crystalline (LC) elastomers under certain conditions of temperature and elongation rate, however. Cold-drawing in LC elastomers, glassy amorphous polymers, and semicrystalline polymers clearly cannot be attributed to common morphological features, but it may have common origins in conformational transitions at the chain level. This investigation is aimed at elucidating the underlying molecular basis for cold-drawing in polydomain smectic LC elastomers. The necking instability is proposed to arise from strong energetic contributions to the elastic free-energy upon elongation, in contrast to the classical picture of entropy-driven rubber elasticity. Elastomers containing larger, more stable domains are hypothesized to exhibit a larger yield stress and to be more prone to mechanical instability due to an increased energetic penalty for disrupting smectic ordering during elongation. Macroscopic mechanical response of polydomain smectic LCE will be linked with conformational instability at the chain level by X-ray measurements of domain size and small-angle neutron scattering (SANS) measurements of chain dimensions. A pressing fundamental issue underlying all of the work is the concept of hairpins, chain folds by which the backbone reverses direction upon itself. The number of hairpins per elastic chain affects the domain size along the layer normal, which will be probed by X-ray lineshape analysis. Using SANS, the number of hairpins per chain will be measured as a function of chain length, temperature, and deformation history. The combined results of X-ray diffraction and SANS will provide potentially transformative insights regarding the role of hairpinned chain statistics in promoting cold-drawing and necking in elastomers, and possibly more broadly in other polymers.
NON-TECHNICAL SUMMARY:
Smectic liquid crystalline elastomers (LCE) are rubber-like materials that possess the flexibility and toughness of a rubber-like polymer, but have layered molecular ordering at the nanometer scale. Smectic LCE have unique mechanical properties that potentially make them useful as vibration damping or impact-absorbing rubber coatings, or as soft actuators with properties similar to muscle tissue. One of their unusual features is cold-drawing, a process by which the material yields and elongates drastically when placed under tension, while forming a contraction or "neck." Most rubber-like polymers do not undergo cold-drawing or necking. To better understand how molecular structure in smectic LCE leads to cold-drawing, experimental methods of small-angle neutron scattering and X-ray diffraction will be applied to characterize structural changes at the molecular level due to mechanical deformation. The results of our study will broaden understanding of the mechanical behavior of rubber-like polymers, and possibly uncover broader insights regarding mechanical instability in polymers. This project supports valuable educational activities at Texas Tech University, including graduate and undergraduate education, ethics training for all researchers involved, and mentoring of postdoctoral researchers. The students and postdocs will actively participate in outreach programs that introduce honors students (including women and minorities) to polymers and Chemical Engineering fundamentals, fostering diversity among future researchers in scientific and engineering disciplines.
Intellectual Merit. The overarching objective of this project was to investigate the molecular basis for mechanical behavior in polymer networks having a smectic liquid crystalline phase. Liquid crystalline (LC) polymer networks and elastomers are rubber-like materials in which the constituent molecules possess limited positional or orientational ordering, forming a liquid crystalline phase. A smectic LC phase is one having layer-like ordering at the nanoscale. LC networks and elastomers have received attention from numerous research groups primarily due to their mechanical behavior, which is dramatically different from that of conventional rubber-like materials. Many fundamental studies have been devoted to understanding the "soft elastic" mechanical response of LC elastomers, while other studies have aimed at triggering actuation through various means by designing soft materials that change shape in response to light, temperature, or electric fields. This project sought to identify the molecular factors that contribute to macroscopic mechanical behavior of LC networks through systematic studies of well-defined smectic networks with "polydomain" morphology. Polydomain networks, which contain numerous, randomly oriented domains, are known to form a contraction or "neck" under tension, undergoing a process called "cold drawing" that is seldom observed in rubber-like polymer networks. It was initially hypothesized that "hairpin" folds in the network strands might account for some of the observed mechanical phenomena. However, X-ray diffraction and mechanical property studies revealed that the mesoscale (10 to 1000 nm) domain structure was a more important factor. The smectic polydomain networks were found to exhibit mechanical characteristics intermediate between those of isotropic rubbers and semicrystalline polymers. Networks with lower crosslinker concentration or long annealing time in the smectic state were more likely to contain large, stable smectic microdomains. These networks exhibited a stronger tendency to undergo neck formation and cold drawing under tension, much like many semicrystalline polymers do above their glass transition temperatures. Networks with high crosslinker concentration and those rapidly cooled from the isotropic state contain smaller, less stable smectic microdomains. These networks exhibited mechanical characteristics that were more elastomer-like. Broader Impacts. This research affects thinking about the mechanical behavior of semi-flexible polymer networks, in which some rigid groups are present in the main chain. Semi-flexible networks having non-Gaussian, "hairpinned" conformations of the network chains differ from isotropic rubbers in that deformation disrupts energetic correlations between neighboring segments, which may result in substantial internal energy changes. The presence of organized microdomains in some systems such as smectics can perturb the mechanical response significantly and impart sensitivity to thermal history, even if these polymers cannot crystallize. These results are more broadly important in understanding deviations from classical rubber-like elasticity exhibited in semi-flexible networks such as segmented polyurethanes. In a practical sense, the results of this study will assist the work of researchers studying soft actuators, as some unexpected ramifications of changing simple parameters such as crosslinker concentration and thermal history have been revealed. Highlights of our work were presented at domestic conferences including the American Institute of Chemical Engineers Annual Meeting (2011, 2012), the Polymer Networks Group Meeting (2012), and the Materials Research Society (2011). International presentations were made at the International Liquid Crystal Elastomer Conference (ILCEC) held in Lisbon, Portugal (2011) and at Shanghai Jiao-Tong University at the Polydomain Liquid Crystalline Elastomer Workshop (2011). Research results were disseminated to the scientific community through three peer-reviewed journal articles. This work had a positive impact on the training and development of human resources, including outreach activities. All participants attended an ethics seminar on scientific misconduct sponsored by Texas Tech, and graduate students completed Responsible Conduct of Research training. Two graduate students and a postdoctoral scholar were trained by the principal investigator in techniques of organic materials synthesis. They gained experience with characterization techniques including transmission electron microscopy, dynamic mechanical analysis, neutron scattering, radiation safety, X-ray diffraction, and gel permeation chromatography, while completing the principal investigator's graduate course in Polymer Processing. An undergraduate student joined our research effort through an Independent Study course as a sophomore and worked several semesters in our lab. The principal investigator and students participated in K-12 outreach activities at Texas Tech that engage the interest of 6th to 8th grade girls and high school students. These programs, which provide an introduction to polymer science among other activities, which are geared toward students who have demonstrated interest in possible careers in science and engineering, were conducted annually in 2010, 2011, 2012, and 2013. This research project offered a clear benefit to society through training and professional development of diverse young scientists for a future workforce that will increasingly deal with advanced materials.
